It is known that the simplest circuit solution for insulating a load from the power supply source is to arrange an inductor in series with the power supply in order to avoid sudden current variations, and a capacitor in parallel to the load, using said capacitor to store electrical charges, as shown in FIG. 1.
As shown in FIG. 1, V.sub.dc is the line supply voltage, L is the insulating inductor, C is the filtering capacitor, R is the resistor that represents the loss of the components, I.sub.L and .delta.I are the static and dynamic currents of the load, and V.sub.out is the supply voltage of the load.
Circuit analysis can be performed by considering the second-order differential equation that represents the circuit. The two solutions of the equation represent the natural frequencies of the network, and in the case of over-damping the two frequencies are real and negative, the response of the network being the sum of two decreasing exponential values.
The physical interpretation of the solutions of the differential equation that represents the circuit of FIG. 1 is as follows. In static conditions, the current of the load I.sub.L flows across the inductor L, while the capacitor C is charged at the voltage V.sub.out =V.sub.dc. When a source of a current .delta.I is applied, the voltage across the capacitor and the current on the inductor cannot change instantly and therefore all the current .delta.I is supplied by the capacitor, causing a gradual decrease in the voltage V.sub.out. The voltage variation then causes current to flow in the inductor. After a long time, the network reaches a new equilibrium, in which all the current flows in the inductor I.sub.L +.delta.I and the voltage returns to the value V.sub.out =V.sub.dc.
The slew rate of the current in the inductor L has the following approximate value: ##EQU1##
The above equation shows that the slew rate is inversely proportional to L and that in order to have low slew rate values it is necessary to use large inductors, on the order of 10 mH-1H, with a considerable area occupation on printed circuit boards.
Accordingly, integrated electronic circuits have been studied and produced which are capable of replacing the inductors while maintaining the same electrical performance as said inductors.
One known circuit solution is shown in FIG. 2, in which the block shown in dashed lines is circuitally equivalent to the inductor L of FIG. 1.
In FIG. 2, the capacitor C.sub.1 acts as a charge accumulator and has the same function as the capacitor C of the circuit of FIG. 1.
The inductor L shown in FIG. 1, which is meant to control the variation in the current absorbed from the power supply, is provided, in FIG. 2, by two transconductors G.sub.1 and G.sub.2, by the differential amplifier A.sub.1, by the resistors R, R.sub.1 and by the capacitor C.sub.2.
The resistor R.sub.L represents the resistance of the load and can vary its value suddenly.
As shown, the circuit of FIG. 2 is constituted by two negative-feedback loops: one is a voltage loop, which sets the voltage across the node V.sub.c1 to the value V.sub.reg, and the other one is a current loop, which sets the current supplied by the transconductor G.sub.1 to the value defined by the load.
The voltage loop provides V.sub.out : EQU V.sub.out =V.sub.reg -R.multidot.I.sub.L (EQ 2)
The current loop provides the current I.sub.out: EQU I.sub.out =I.sub.L .multidot.gm.sub.1 .multidot.R.multidot.gm.sub.2 .multidot.R.sub.1 (EQ 3)
If gm.sub.1 =R.sup.-1 and gm.sub.2 =R.sub.1.sup.-1, one obtains I.sub.out =I.sub.L.
The slew rate .DELTA.I.sub.out /.DELTA.t can be calculated assuming a sudden variation in the current of the load.
If at a certain instant the current I.sub.L varies by the amount .DELTA.I.sub.L, at the output of the transconductor G.sub.2 one obtains a current .DELTA.I.sub.sense =R*.DELTA.I.sub.L *gm.sub.2 supplied to the capacitor C.sub.2 and the voltage V.sub.c.sub.2 varies according to the following rule: ##EQU2## providing in output from the block G.sub.1 a current variation equal to: ##EQU3##
In view of the choices made for gm.sub.1 and gm.sub.2, i.e., the transconductances of the transconductors G.sub.1 and G.sub.2 respectively, the following relation is derived: ##EQU4##
Once the resistor R.sub.1 is defined, the slew rate is a function of the current variation of the load .DELTA.I.sub.L and of the value of the capacitor C.sub.2.
Once the capacitor C.sub.1 is defined, the capacitor C.sub.2 is chosen so as to ensure the stability of the two feedback loops.
Small-signal analysis of the circuit of FIG. 2 shows that there are two poles, due to the presence of the two capacitors, whose pulses are: ##EQU5##
In general, R.sub.L is much higher than R but lower than R.sub.1, while C.sub.1 is higher than C.sub.2 and this can cause instability of the network. In order to avoid oscillation problems, the gain of the loop is lowered so as to obtain an acceptable phase margin (m.phi.=30.degree.-40.degree.). This is achieved by reducing the gain of the differential amplifier A.sub.I.
Therefore, the known circuit shown in FIG. 2 has drawbacks which limit its use.
First of all, the precision of the DC output current depends on the construction of the blocks G.sub.1 and G.sub.2 and on the coupling of the resistors R and R.sub.1.
The slew rate is a function of the load and of the capacitor C.sub.2.
Moreover, the value of the voltage V.sub.out (set to the voltage V.sub.c1) is a function of the current of the load. The choice of the capacitors C.sub.1 and C.sub.2 must be such as to not trigger oscillations in the network. Finally, the gain of the differential amplifier A.sub.1 must be chosen as a function of the optimum phase margin.